The epidemic of obesity is closely associated with increases in the prevalence of diabetes, hypertension, coronary heart diseases, cancer and other disorders. The role of white adipose tissue is to store lipids, and it is associated with obesity. The role of the brown adipose tissue (“BAT”) is effectively the opposite. It is specialized in lipid combustion and the dissipation of energy as heat. Indeed, the brown adipocyte contains lots of mitochondria (in which cellular combustion occurs) and uniquely expresses BAT uncoupling protein-1 (“UCP1”). UCP1 acts as an uncoupler of oxidative phosphorylation, resulting in dissipation of energy as heat. The sympathetic nervous system stimulates mitochondriogenesis and UCP1 expression and activity. BAT-associated thermogenesis in rodents is increased upon exposure to low temperature (e.g., preventing hypothermia) or as a result of overeating, burning excess absorbed fat and preventing weight gain. BAT, by modifying susceptibility to weight gain and by consuming large amounts of glucose, also improves insulin sensitivity. It therefore plays an important role in the maintenance of body temperature, energy balance and glucose metabolism.
Experiments with transgenic animals support the potential anti-obesity properties of BAT. For example, the genetic ablation of BAT has been reported to cause obesity, while genetic increase in the amount and/or function of BAT (and/or UCP1 expression) reportedly promotes a lean and healthy phenotype. Specifically, mice with a higher amount of BAT gain less weight and are more insulin-sensitive than control mice. Recently, ectopic BAT depots were evidenced in the mouse muscle, which were proposed to provide a genetic-based mechanism of protection from weight gain and metabolic syndrome.
Although UCP1 is reported to play a role in the control of energy balance in rodents and UCP1-expressing BAT is present in human neonates, it has long been thought that there was no physiologically relevant UCP1 expression in adult humans. Indeed, UCP1-expressing BAT was thought to disappear early in life, and adult humans were thought to be devoid of BAT.
As such, a need exists to carefully identify and study ways to provide more BAT in the adult body and/or stimulate UCP1 expression, for the study, prevention and treatment of various metabolic diseases such as obesity, type 2 diabetes, insulin-resistance dyslipidemia and type 1 diabetes.